Respiratory systems provide breathable gas, such as oxygen, anesthetic gas and/or air directly to a patient's mouth, nose or airway to assist or facilitate breathing by the patient. A ventilator may be used as part of the respiratory system to drive the breathable gas to the patient through an inspiratory limb hose or conduit. An expiratory limb hose or conduit may be provided to allow air to expel from the patient.
It is typically desired to warm and impart humidity to the breathable gas before it is provided to the patient. For that purpose, many respiratory systems include a heater system having a chamber for holding water and a heater unit supporting a heater, which may be comprised of one or more heating elements and a metal plate defining a hot plate. A wall of the chamber, such as the bottom surface of the chamber, is thermally conductive. The chamber is removably supported on the heater unit with the bottom surface in thermal contact with the hot plate of the heater to thus heat the water in the chamber. The chamber may be manually refillable, or there may be a water source to selectively fill the chamber as it empties. The breathable gas is coupled to the chamber and is passed through the chamber to be heated and humidified. An example of a heater and chamber arrangement is shown in U.S. Pat. Nos. 6,988,497 and 5,943,473. The inspiratory limb carries the heated and humidified gas to the patient. The inspiratory and/or expiratory limbs may also be heated such as by heater circuits comprised of heater wires running through and along the hose or conduit interior.
The heater unit typically also houses several other heat generating components, such as an electrical transformer, controller circuitry, and power switches (which may be thermally coupled to a heat sink), utilized to regulate the temperature of the heater and provide power to regulate heat input to the limbs from the heater circuits thereof. These components are disposed within the heater unit and consequently heat the air therein which then needs to be cleared from the heater unit to avoid overheating. Otherwise, the performance and life expectancy of the heater unit may be adversely affected. It is known to provide a fan within a housing to force air through the housing in order to cool the air and/or components therein. Fans are not desired for heater units of a respiratory system, which instead rely principally on flow of air therethrough from natural convection cooling. By way of example, a heater unit may have one or more air inlets adjacent to or contained within the bottom of the heater unit, and one or more air exhausts adjacent to or contained within the top of the heater unit, such that as the heated air therein naturally rises, cooler air will be brought in through the air inlets and the hotter air will exit from the air exhausts.
Use of natural convection cooling is thus helpful to remove heated air, but there are some disadvantages with prior designs. Further, evolving design requirements for heater units used in respiratory systems impose increased control functionality requirements and more stringent response times, all of which create additional heating load within the heater unit. But hospital and other constraints may limit the general size of the heater unit, such that there could be insufficient cooling.